Abstract

Modification of protein by carbonyl compounds
under in vitro physiological conditions is sitedirected.
There are few reports of the site specificity of
glycation of proteins using heating conditions of relevance
to food processing. The aim of this study was to determine
the site specificity of modification of b-casein (bCN) by
glucose and methylglyoxal (MGO). bCN (1.33 M, 3.2%)
was heated with either glucose (1.345 M, 4.6%) or MGO
(1 mM) at 95C for up to 4 h. Tryptic digests were
prepared and analysed by ultra performance liquid chromatography electrospray ionisation mass spectrometry
(UPLC-ES/MS). The sites of formation of the Amadori
product, Ne-(fructosyl)lysine (FL), and the advanced glycation end-products, Ne-(carboxymethyl)lysine (CML),
MGO-derived dihydroxyimidazolidine (MG-DH) and
MGO-derived hydroimidazolone (MG-HI), were located.
FL and CML were detected at K107 and K176 residues in
bCN/glucose incubations. Indigenous Ne-(lactulosyl)lysine
was detected at K107 only. MG-DH and MG-HI were
detected at R202 and possibly R183 residues in both bCN/
glucose and bCN/MGO incubations. Glycation of bCN by
glucose and MGO resulted in similar site specificity for
MG-DH and MG-HI formation.